All the curling stones used in every Olympics have come from the same small island

Few people quite understand what exactly curling is, but every four years, people across the world suddenly find themselves invested in a sport that, at first glance, can be described as people pushing rocks across ice with brooms.

For those who are using this year’s go-around to learn what they can about the sport, here’s a fun fact to tell at the next watch party: Olympic curling rocks aren’t just any old bits of earth; they all come from the exact same kind of stone from the exact same place.

The island — Ailsa Craig, also known as “Paddy’s milestone” — is a volcanic plug, meaning it coalesced over an extinct volcano, apparently leaving the granite in the perfect condition to make curling stones. All the stones used during the Olympic Winter Games are produced by the only company with rights to the Ailsa Craig granite: Kays of Scotland, which has been creating the stones since 1851. According to the Huffington Post, thousands of tons of two varieties of stone are removed from the ground once every decade: a blue hone granite, which is impenetrable by ice and water and makes up the insert and running band of the curling stone, and a green granite that composes the body of the stone. There is apparently a third variety, red hone granite, but it isn’t used in curling stones.